David Hughes is the Daily Telegraph's chief leader writer. He has been covering British politics for 30 years.

David Cameron flashes a smile and a stiletto in PMQs

A game Harriet Harman is beginnng to wilt a little at PMQs, hardly surprising given David Cameron's easy mastery at the dispatch box. Today was a perfect example. Labour's acting leader homed in on Ken Clarke's speech last week in which he questioned the effectiveness of short prison sentences. Harman said that such sentences were essential when tackling domestic violence. She then reached into her file of pre-prepared quips to make the point that while his coalition partner Nick Clegg might support such a policy, his mother – a stalwart of the Newbury magistrates bench – did not. Mrs Cameron had spoken of the usefulness of short sentences. Sometimes, said Hattie (pause for drumroll), people should listen not to their partners but to their mothers (wonder what her hubbie, Jack Dromey, thinks of that). She got her laugh – but not nearly as big as the one Cameron got with his lightning return of serve when he pointed out that his mother had always told him that she had to hand out such sentences "to badly behaved CND protesters outside Greenham Common". When Harman then tried to nail the PM on police numbers, away went the smile and out came the stiletto, if you'll forgive the pun. "At least I will not be wandering around my constituency in a stab-proof vest", Cameron snapped, a reference to one of Harman's less inspired phot ops. Game, set and match.